The only reason i have to speculate that Ace is at level 6 is that Oda would probably want us to explore each level of ID before he leaves. On the other hand what if ace was overhyped? I mean he fought with WBs name. I can see that many people were scared of him until BB came and pwned him. I mean what if his threat is so that only level 5 suits him? What if his threat isn't so great that he would have been put at level 6?
Well I don't think Oda is going to make this a 6 level prison just to have Luffy get caught in Level 4. Not show us Level 5 or level 6. The floor that Ace isn't on. Especially when he has hyped it up as the floor where monsters hidden from history are kept. He'll go through all 6 floors or what max floor this place has by the time this arc is done. Of course Ace being on level 6 like Buggy foreshadowed in his rumor would be the logically reason why Luffy would go up to Level 6.
As for Ace. No he's not fucking overhyped. He should be on the bottom floor. He is WB 2nd in Command. His capture was enough of a accomplishment by the WG that it was enough to give Blackbeard a Shichibukai title when he had a bounty of zero and has given the WG a reason to go to war with WB. He was a catalyst to this gigantic incident. Someone that has given the WG enough balls to think that yeah we can trap WB and actually win. So to say that he's overhyped is ridiculous. The WG actions speak for themselves about how high they regard Ace's capture.